Optical-fiber attenuators are used for optical signal-power reduction, mainly in short distance telecommunication links or in long distance trunk-lines with signal repeaters having a fixed amplification. In a network, whenever the optical signal power is higher than the dynamic range of the detectors (normally less than 25 dB), the optical signal needs to be lowered by attenuating the signal power to avoid saturation of the detectors.
There are mainly three different types of designs of optical-fiber attenuators: mechanical attenuators, see A. Benner, H. M. Presby. and N. Amitay, "Low-reflection in-line variable attenuator utilizing optical fiber taper," J. of Lightwave Technology, Vol. 8, No. 1, pp. 7-10, January, 1990, laser diode attenuators, see N. Kashima, "A new approach to an optical attenuator for a time compression multiplex system using a laser diode as both transmitter and receiver", J. of Lightwave Technology, Vol. 9, No. 8, pp. 987-990, August, 1991, and fused attenuators, see e.g. M. Cork, "Passive fiber optic components," Short Course Notes, OFC'91, San Diego, p. 63, February, 1991.
A typical mechanical attenuator is shown in FIG. 1. Two fiber ends 1 having claddings 2 surrounding cores 3 are made broader at the end surfaces, in the portions 5, producing a taper 7 as seen from the end surfaces. A stabilizing sleeve 9 keeps the ends in place with the end surfaces at some predetermined distance. There are several variations in the design, e.g., with or without the taper 7, the end surfaces may extend perpendicularly or non-per-pendicularly to the longitudinal direction of the fibers, there may be a piece of absorption or half-reflection glass in the air-gap 11, etc. The advantage of a mechanical attenuator is that the attenuation can be adjusted in a mechanical way by moving the end surfaces of the fiber ends, see the arrows 13. However, if the attenuator is to be used as a line build-in component having a fixed attenuation in a system, the variation of attenuation with varying temperature, a high production and maintenance cost, the high reflection--low return loss of about 10-40 dB--will altogether make a system comprising mechanical attenuators very unstable and expensive.
A typical fused attenuator, see FIG. 2 and the article by M. Cork cited above, can be manufactured by offset splicing, as described in the published International patent application WO-A1 95/24665 "Controlled splicing of optical fibers" corresponding to the Japanese patent application 523,400/95 and U.S. patent application Ser. No. 08/400,968, which is incorporated by reference herein. Here the fiber ends 1 are only displaced in a lateral direction, an offset x existing between the sides of the fibers, that is between the outer surfaces of the cladding 2 as seen in a chosen direction. When the splice is made, the surface tension effect will try to align the outer surfaces of the claddings, this also producing a curved end segment 15 of the fiber cores 3 at the formed end surfaces, at the corresponding splice surface. This type of attenuator has a very low production cost, has a very high attenuation stability and reliability, and almost no reflection is obtained from the attenuator. No maintenance is needed. However, the offset of the fiber claddings may not be quite satisfactory, as considered by some system designers. They are worried about the mechanical stress concentration at the splice point when the attenuator is used during extended time periods, especially when the splice is only re-coated without using a protection sleeve. These stress concentrations may lead to unwanted cracks of the fiber claddings or even ruptures of the fiber joints.
The British patent application GB-A 2 128 766 discloses a single mode optical fiber attenuator, wherein the ends of two optical fibers are fused together by applying heat to the ends. When the ends have been satisfactorily fused together the heating is continued. This causes the core region to partially diffuse into the cladding material. When the fiber is then cooled an attenuator is created, i.e. light propagating in the fiber is attenuated as a result of the non-distinctiveness of the fiber cores due to the partial diffusion. The heating can be prolonged during quite a long time. Times of 70 and 150 seconds are mentioned for making attenuators of 4 dB and 8 dB respectively.
In the article "Splice loss of single mode fiber as related to fusion time, temperature, and index profile alteration", by J. T. Krause, W. A. Reed and K. L. Walker, IOOC-ECOC '85, pp. 629-632, the loss or attenuation of a single mode fiber splice is discussed and in particular it is stated that the loss is dependent on the offset of the fiber cores and on the diffusion of the core material. U.S. Pat. No. 4,557,556 for George A. Decker discloses a method for fabricating an optical attenuator wherein the axes of two optical fiber ends are misaligned by an offset distance and then the fiber ends are melted to each other. While the fiber ends are melted, the ends are moved since the surface tension tends to align the exterior sides of the claddings of the optical fibers and then also the axes of the cores will be essentially aligned. The same method is disclosed in German Offenlegungsschrift DE-A1 42 36 807. A similar method is discussed in the European published patent application EP-A2 0 594 996.